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What Makes An Airplane Fly - Level 1

For an airplane to fly, it must always engage in a tug of war between the opposing
forces of lift versus weight and thrust versus drag.

For a moment, think of an airplane moving from right to left and the flow of air moving
from left to right. The weight or force due to gravity pulls
down on the plane opposing the lift created by air flowing over the wing.
Thrust is generated by the propeller and
opposes drag caused by air resistance to the airplane. During take
off, thrust must be greater than drag and lift must be greater than weight so that the
airplane can become airborne.

For landing thrust must be less than drag, and lift must be less than
weight.

THE FOUR FORCES ACTING ON AN AIRPLANE

An airplane in flight is the center of a continuous tug of war between fourforces: lift, gravity force or weight, thrust,
and drag. Lift and Drag are considered aerodynamic forces because
they exist due to the movement of the aircraft through the air. The weight
pulls down on the plane opposing the lift created by air flowing over the wing. Thrust is
generated by the propeller and opposes drag caused by air resistance to the frontal area
of the airplane. During take off, thrust must overcome drag and lift must overcome the
weight before the airplane can become airborne. In level flight at constant speed, thrust
exactly equals drag and lift exactly equals the weight or gravity force. For landings
thrust must be reduced below the level of drag and lift below the level of the gravity
force or weight.

Lift

Lift is produced by a lower pressure
created on the upper surface of an airplane's wing compared to the pressure on the wing's
lower surface, causing the wing to be "lifted" upward. The special shape of the
airplane wing (airfoil) is designed so that air flowing over it will have
to travel a greater distance faster, resulting in a lower pressure area (see illustration)
thus lifting the wing upward. Lift is that force which opposes the force of gravity (or
weight).

Many believe that this explanation is incorrect because flat wings (such as seen on
balsa wood airplanes, paper planes and others) also have managed to create lift.
Please read How planes fly: the physical description of
flight as well to get a fuller understanding of the creation of lift. It is at a
more advanced level, though.

Thrust

Thrust is a force created by a power source which gives an airplane
forward motion. It can either "pull" or "push" an airplane forward.
Thrust is that force which overcomes drag. Conventional airplanes utilize engines
as well as propellers to obtain thrust.

Drag

Drag is the force which delays or slows the forward movement of an
airplane through the air when the airflow direction is opposite to the direction of motion
of the airplane. It is the friction of the air as it meets and passes over and about an
airplane and its components. The more surface area exposed to rushing air, the greater the
drag. An airplane's streamlined shape helps it pass through the air more easily.

If you want a more advanced analysis of what makes an airplane fly, look in Level 2 and Level 3 for more
information.